There are a variety of commercially available wireline formation testing tools used for determining the pressure and mobility of fluid trapped in a subterranean formation, which is typically referred to as a reservoir. A typical prior art wireline formation tester employs an elongated body adapted for downhole operation in a borehole that traverses a reservoir. A probe is deployable from the elongated body. The probe includes a deformable packer surrounding a flow line. The flow line extends into the elongated body. A pretest chamber is disposed within the elongated body in fluid communication with the flow line. A hydraulically-driven (or electrically-driven) piston moves within the pretest chamber. A pressure sensor measures fluid pressure of the flow line. Proper placement of the formation tester requires lowering the elongated body of the formation tester into the borehole and deploying the probe from the elongated body such that the probe interfaces to borehole wall.
Boreholes can be drilled with the borehole fluid pressure higher than the pressure of the formation fluids. This is called over-balanced drilling. Boreholes can also be drilled with the borehole pressure less than the pressure of the formation fluids. This is called under-balanced drilling.
In the case of the borehole drilled with over-balanced drilling, the borehole wall may be coated with mud cake that is formed from the circulation of drilling fluids through the borehole during drilling operations. The mud cake provides a seal (typically referred to as a mud cake seal) that isolates borehole drilling fluids from the reservoir fluids trapped in the formation adjacent the borehole in order to reduce the quantity of drilling fluid that flows from the borehole into the adjacent formation. Pressure and mobility of the reservoir fluids are measured with a procedure that includes a “draw-down” procedure followed by a “build-up” procedure. Before the “drawdown” procedure, the deformable packer is pushed against the mud cake and provides a seal between the packer and the mud cake, and the probe is pressed against the mud cake on the borehole wall. During the “drawdown” procedure, a small amount of formation fluid is extracted from the formation into the flow line of the probe. The purpose of the draw-down procedure is to establish fluid communication between the flow line and the formation fluid. Drawdown includes moving the piston in the pretest chamber to expand the volume of the flow line so that the pressure of the fluid in the flow line is reduced sufficiently below formation pressure causing the mud cake seal to break. The purposes of measuring the pressure of the fluid in the flowline during and after the drawdown are to verify good fluid communication between the flow line and the formation and to verify good fluid isolation between the flow line and the borehole fluid. During the “drawdown” procedure, the pretest piston is configured in a “retracted” position in the pretest chamber and the fluid in the flow line (including the pretest chamber) is at a pressure below the pressure of the reservoir fluid trapped in the formation adjacent the probe. The “build-up” procedure establishes pressure equilibrium between the fluid in the flow line and the reservoir fluid during a build-up time period. During the “build-up” procedure, the pretest piston remains stationary in the “retracted” position in the pretest chamber. Reservoir fluid flows from the formation into the flow line because the formation fluid pressure is higher than the fluid pressure in the flow line. Continued inflow allows the fluid pressure in the flow line to build up until equilibrium is established. When equilibrium is established, the fluid pressure in the flow line equals the formation fluid pressure (the fluid pressure of the reservoir fluids trapped in the formation adjacent the probe). The changing pressure in the flow line is monitored by the pressure sensor of the tool. Mobility of the formation can be calculated from the pressure of the flow line fluid during drawdown or during drawdown and build-up. Mobility is defined as the ratio of formation permeability (in unit Darcy) to formation viscosity (in Poise).
The pressure signal is needed during both drawdown and buildup in order to calculate mobility. In the case of the borehole drilled with under-balanced drilling, there is no mud cake seal and only mobility of the reservoir fluids is measured with a “drawdown” procedure and a “build-up” procedure. Before the “drawndown” procedure, the deformable packer is pushed against the borehole wall and provides a seal between the packer and the borehole wall, and the probe is pressed against the borehole wall. In the “drawn down” procedure, the pretest piston of the tool is configured in a “retracted” position in the pretest chamber and the fluid in the flow line (including the pretest chamber) is at a pressure below the pressure of the reservoir fluid trapped in the formation adjacent the probe. The “build-up” procedure allows for the reservoir fluid to flow into the flow line and establish pressure equilibrium between reservoir fluid and the fluid in the flow line during a build-up time period. During the “build-up” procedure, the pretest piston remains stationary in the “retracted” position in the pretest chamber. The changing pressure in the flow line is monitored by the pressure sensor of the tool. Mobility of the formation can be calculated using the drawdown pressure data or using the drawdown and the build-up pressure data measured by the tool. Mobility is defined as the ratio of formation permeability (in unit Darcy) to formation viscosity (in Poise).